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Serial Data Transmission with CompactPCI Serial

CompactPCI Serial (PICMG CompactPCI-S.0) is the successor to CompactPCI. This standalone specification is based on pure serial architecture and offers data transmission speeds of up to 32 GB/s. The application bandwidth is therefore even greater than was the case with CompactPCI.

Soft migration

To avoid presenting a sharp edge to CompactPCI, a sub-specification to the CompactPCI base specification, CompactPCI PlusIO (PICMG 2.30), was developed. This allows a soft migration from CompactPCI. CompactPCI PlusIO defines a unified pinout on the P2 connector of the 32-bit CompactPCI system slot on which the four new serial buses, PCIe, GbE, S-ATA and USB are defined. It is thus possible to create hybrid CompactPCI/CompactPCI Serial systems (Fig. 1). The CPU addresses the parallel CompactPCI slots in the system on the P1 connector, while the P2 area is connected to the fast CompactPCI Serial slots. As a result the transfer rate is increased above that of the parallel bus by a factor of 300. Instead of the roughly 150 connections of CompactPCI, with CompactPCI Serial there are more than 360, of which 340 are high-speed connections. System components such as slow I/O boards can remain on CompactPCI. As a result, only special boards that require one of the new protocols or more bandwidth need to be converted to the new technology.

Backplanes for CompactPCI Serial and hybrid systems

Schroff has developed backplanes and complete systems for both specifications. The backplanes available include, for example, 8-slot hybrid backplanes with three CompactPCI peripheral slots, one CompactPCI PlusIO system slot and four CompactPCI Serial peripheral slots (Fig. 2). These backplanes are available in two versions - with and without rear I/O on the CompactPCI Serial slots. They represent the maximum configuration for hybrid systems and can thus be used in all possible applications. Other backplanes with fewer slots are currently in preparation, such as a 5-slot version.

Likewise for CompactPCI Serial, Schroff has also first developed backplanes for the maximum configuration with nine slots, one system slot and eight peripheral slots. This again allows the widest range of applications to be accommodated. There are even different backplane versions in which PCIe, USB and S-ATA are provided in single star in all cases. One version also features a single star for gigabit Ethernet, while the other version has full-mesh wiring in the GbE area and is thus best suited for building processor farms or clusters. Both of these versions are available with or without rear I/O.

CompactPCI Serial backplanes have twice the number of signals and less space between connectors, and thus offer a high performance density. The signal lines are thus necessarily very close to one another, and excessive crosstalk can occur. Crosstalk, like reflections, contributes to noise and jitter. The skill of the designer is in reaching the optimum compromise between performance density and the number of layers, while generally reducing impedance discontinuities. 3D modelling of the circuit board elements and simulation of the complete transmission channel to determine the bit error rate is a very helpful method of designing boards and backplanes at the first draft with a sufficiently low bit error rate.

Mechanics and power supply

The mechanical construction of CompactPCI Serial systems (Fig. 3) matches that of Schroff's 8-slot 3+1 CompactPCI system with an integrated 1 U fan drawer. Fitted with a backplane, up to nine 3 U boards can be accommodated and the system slot is positioned on the left (Fig. 2). It is also possible here to use double-width CPUs without any design modifications. There is sufficient space for e.g. hard disk drives to be fitted. Additionally, the rear section includes a space for rear I/O boards.

Customers may opt for one of two types of power supply unit. The 19" PSU (3 U, 8 HP, 160 HP) with an output of 250 W and rear-mounted IEC mains input has even narrower tolerances at the limit values for currents, voltages etc. than is called for by the specification. This is of particular benefit where multicore processors are present. For custom applications this PSU is also available in a 48 V version. As a lower-cost alternative, an ATX unit from Schroff's latest ATX PSU family can be fitted. Despite the larger temperature range of these ATX PSUs, from -10 to +70 °C, their efficiency is over 75 %. Overvoltage protection is also relatively high at 4 kV. MTBF values are about 100,000 h at 50 °C. This is achieved partly through the use of ball-bearing PSU fans. The fan uses a tacho signal to regulate speed and thus also the noise level in relation to the ambient temperature. Fault or failure of the fans is signalled direct to the boards through an interface. The capacitors used, rated to up to +105 °C, also offer a long service life.

Optimal cooling of the system is assured by a hot-swap fan drawer under the board cage that pulls out to the front. This can optionally be upgraded with a filter mat. Depending on the PSU selected, the drawer is fitted with either two or three fans that can be quickly replaced when necessary.

Possible applications

CompactPCI Serial, combined with the CompactPCI PlusIO defined as a migration path, are already showing a major market potential. Users changing to the new technology from CompactPCI have very diverse reasons for doing so. An application example in freight traffic shows a hybrid backplane with CompactPCI PlusIO system slot used in conjunction with a conventional CompactPCI backplane in one system. Here the hybrid backplane serves to accommodate future system upgrades so that faster CompactPCI Serial boards can later be used.

Another application, in industrial automation, links a system via WLAN to the company network. In this arrangement the USB interface is used that is defined on the CompactPCI Serial slots. A USB WLAN stick is mounted on a CompactPCI Serial board and can be linked directly via the backplane to the CompactPCI PlusIO processor board using the USB protocol. This represents a simple and straightforward solution for linking the system to a network via WLAN.

In some railway applications and in industrial automation, hybrid backplanes are also beginning to be used for integrating hard disk boards simply into the system. CompactPCI does not allow plug-in hard disk modules to be directly integrated, since in CompactPCI only the PCI bus is implemented. However, current hard disk drives use the S-ATA or SAS protocols. CompactPCI PlusIO and CompactPCI Serial define both protocols on both slots, so that a hard disk drive on a periphery board can be connected to the system simply. This allows powerful Raid systems to be simply constructed.

Another example application of CompactPCI Serial is a special use in audio processing. This makes use of the full-mesh topology of the Ethernet defined in the J6 area of CompactPCI Serial. This allows audio data to be exchanged extremely fast between the individual digital signal processors. The free user-defined pins in the J2 to J5 range serve for future upgrades. Here the full-mesh topography of the J6 area is mirrored in order to increase the data volume further.

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